Emperor Dragonfly

Anax imperator (Leach, 1815)

Family: Aeshnidae - Hawkers

Order: Odonata (Anisoptera) - Dragonflies

Class: Insecta

Phylum: Arthropoda

Kingdom: Animalia

Red List status: Least Concern

Total length: 66-84 mm

Abdomen length: 50-61 mm

Hindwing length: 45-52 mm

Larval body length: 44-56 mm

Other common names: Blue emperor

DESCRIPTION

Adult: A large, robust, long-bodied dragonfly with a 'waisted' appearance to the abdomen, which is longer than the hindwing. The eyes range from green to blue, with yellow to green undersides. Animals have a plain green thorax and, in males, a light blue abdomen (except for the first segment, which is usually green) with a broad black dorsal stripe. The intensity of the blue colouration varies with weather conditions, becoming paler in cooler weather.

 In profile view, there is a black marking at the base of each segment. Immature males and females typically have a green (sometimes brown in immatures) abdomen, with a distinct thin yellow ring at the base of the second abdominal segment only in newly-emerged individuals, becoming green with age. The dorsal midline is often more brownish than black in females and tenerals. In both sexes, the costa (leading edge of the wing) is yellow, and the pterostigma brown. The wing itself is otherwise clear, but can become brownish in older females. The frons is marked with a black pentagon at the base, and there is a blue bar in front of this marking. Some females have a blue abdomen; they can be distinguished from males by two triangular blue markings on top of the thorax, just in front of the wings.

Larva: A very large, elongate dragonfly larva, with a torpedo-like body and large, round eyes characterised by eye length much greater than width. The rear margin of the head is straight, and the head as a whole has a roughly circular appearance. The labium is three and a half to four times as long as wide. Dorsal spines are present on the seventh and eighth abdominal segments, but there is no spine on the sixth segment. Early-stage (first year) larvae are banded in black and white, colouration that is lost with age. This may be an adaptation that camouflages them against older larvae of the same species by imitating the effects of incident light near the water surface.

Similar species: The combination of green thorax and blue abdomen distinguishes males (and some females) from most other dragonflies. The all-green, non-metallic patterning of females is characteristic. Among remaining species, the blue-eyed hawker (Aeshna affinis), green hawker (Aeshna viridis) and southern hawker (Aeshna cyanea) have a predominantly black, banded abdomen with blue and/or green spots, lack a dorsal stripe and have distinct black markings on the thorax. Lesser emperor (Anax parthenope) has a brown thorax and a yellow ring at the base of the second abdominal segment that persists into adulthood. Occasional individuals of vagrant emperor (A. ephippiger) may have blue abdomens, but never have blue eyes and the thorax is brown. Common green darner (Anax junius) may be encountered as an occasional vagrant from the Americas in Western Europe; this species lacks blue eyes, never has blue markings on the thorax, and has a distinct frons marking resembling a bulls-eye, with a circular, black central mark surrounded by a blue ring.

Larvae can be distinguished on the basis of their shape and large size. Hawkers in the genus Aeshna have eyes that are wider at their base than they are long; the hairy dragonfly (Brachytron pratense) has larvae with rather small eyes. Compared with lesser emperor (Anax parthenope), the spines at the side of the ninth abdominal segment are slightly longer, but these two species can only be reliably be distinguished by examining the genitalia. In males, the projection at the base of the epiproct is as long as it is broad in A. imperator, but half as long as broad in A. parthenope. Females are more difficult to distinguish, but it has been suggested that the ovipositor (on abdominal segment 9) is larger in relation to the width of that segment in the emperor (ratio 0.76 vs. 0.6 in A. parthenope) (Cham, 2007).

HABITAT

This is a species of still and occasionally slow-flowing waters, most often encountered around larger, well-vegetated waterbodies. It is known to colonise newly-formed pond, and is able to tolerate brackish conditions. Larvae inhabit pondweed.

Reproductive habitat: Commonly floating vegetation away from the banks of the pond.

 

BEHAVIOUR

Males of this fast-flying species patrol open areas of the waterbody, often higher and further from shore than related species. When they come into land, individuals typically rest vertically, low down in vegetation. In flight, the animal may hold its abdomen so that it curves downwards. Animals are strongly territorial and will chase off smaller rivals, including members of other species with a similar appearance.

Emperors will typically eat prey on the wing, but large prey items, such as large butterflies, dragonflies and similarly-sized insects, will often be consumed from a perch.

Larvae are active predators that respond rapidly to movement and use their well-developed eyes to track prey. They are typically well-camouflaged within vegetation, clasping twigs or stems close to the water surface. Unusually, they have been known to hunt in open water, using jet propulsion to pursue prey.

Breeding behaviour: In contrast to other European emperors (genus Anax), females lay eggs alone, ovipositing directly into floating vegetation, including deadwood, away from shore. This exposed situation makes animals highly visible compared with related species.

Emergence: Emergence takes place between April and September; in the UK, the peak emergence period is from May to July. Larvae often emerge synchronously following a sequence of warm nights, and will typically select an emergence support a day or more in advance of moulting. During this period, animals may travel some distance from water, usually within 6 m but as far as 30 m has been recorded. Larvae may climb up to 5 m into trees in preparation for emergence. Emergence itself takes around three hours; once the wings have fully expanded and are ready for use, animals may then wait for suitable early morning light levels before taking off, whirring their wings to increase their body temperature sufficiently for take-off in cool conditions. If conditions are particularly poor, partially-moulted larvae may actually return to water to reduce predation risk on land during the day, before re-emerging the following evening.

Flight season: Highly dependent on location, with North African populations flying from March until December. In northern Europe, animals are most abundant from June to August.

 

BIOLOGY

Life cycle: Eggs hatch three weeks after being laid. In contrast to most members of the genus Anax, at least in the UK, development follows immediately from oviposition with no intervening period of dormancy. Larval development takes either one or two years, partially depending on when eggs were laid. Following emergence, maturation takes between a week and 12 days in males, 13-16 days in females.

Lifespan: Following emergence, captive emperor dragonflies have been known to survive for up to 80 days.  

 

STATUS

This common and familiar dragonfly in Europe is nevertheless a comparatively recent colonist from Africa, and still rapidly expanding its range northwards (Dijksta & Lewington, 2006). In the UK, it is absent from upland areas.

 

REFERENCES

 

Cham, S. 2007 Field Guide to the larvae and exuviae of British Dragonflies Volume 1: Dragonflies (Anisoptera) The British Dragonfly Society: 76 pp.

Corbet, P. and Brooks, S. 2008 Dragonflies New Naturalist Series 106, Harper-Collins, London: 456pp

Dijkstra, K-D.B. and Lewington, R. 2006 Field Guide to the Dragonflies of Britain and Europe British Wildlife Publishing: 320 pp

Smallshire, D. and Swash, A. 2010 Britain's Dragonflies: A field guide to the damselflies and dragonflies of Britain and Ireland 2nd Edition WILDGuides: 208 pp

Anax imperator female, blue form, ovipositing. Note the yellow costa, thick mid-dorsal line and the green of the thorax and first abdominal segment. Unusually, this female exhibits blue triangular marks in front of the forewing, a feature typical of males, but the animal's behaviour identifies its sex.

Blue female emperor dragonfly ovipositing, Buckinghamshire

RANGE

All North Africa and Europe north to Denmark on the mainland, and through southern Britain and Ireland Populations become scarcer and more scattered in the north of this range and inland from the coast in Ireland. In East Africa. the species occurs south as far as Zimbabwe and Madagascar.

Range        Description        Habitat        Behaviour        Biology         Status        References